A suspension device is known in the art which varies the roll rigidity of a vehicle according to the turning state in order to improve the turning behavior of the vehicle.
Tokkai Hei 7-215035 published in 1995 by the Japanese Patent Office discloses a method to decrease the roll rigidity of the front wheels and improve the grip force of the front wheels relatively in a turn transient state, and to increase the roll rigidity of the front wheels and improve the grip force of the rear wheels relatively in a turning steady state. The turn transient state is a state wherein a steering angle is changing, and the turning steady state is a state wherein the steering angle is held almost constant when turning.
In this device, the damping force of hydraulic cylinders between the front wheels and the vehicle body is increased and decreased to adjust the roll rigidity. When the damping force of the hydraulic cylinders decreases, the roll rigidity of the front wheels decreases, and conversely when the damping force increases, the roll rigidity of the front wheels increases.
However, in the aforesaid method, when a sequential steering operation (to the left followed by a steering operation to the right or vice versa) is performed on series curves and the turn transient state continues, the damping force of the hydraulic cylinders maintains the decreased state and does not recover.
Therefore, due to a roll moment which repeatedly acts on the vehicle body, the front of the body tends to roll to the left and right.